DESCRIPTION (provided by investigator): There are two key steps involved in the ability of anthrax toxin to target and kill mammalian cells. The first step involves binding to the mammalian cell surface through the recently cloned Anthrax Toxin Receptor (ATR). The second step involves proteolytic cleavage of the anthrax toxin-PA (protective antigen) residue by a ubiquitously expressed mammalian cell protease furin, such that it now is able to form a multimeric complex (pore) that enables entry of the LF (lethal factor) and EF (edema factor) subunits of anthrax toxin into the cell. The advantage of targeting these two steps for the development of anti-toxin strategies is that a single hit would neutralize both the LF and EF mediated toxicities. The validity of these two steps as targets comes from recent work that used a soluble ATR receptor (to prevent binding to cells) to protect cells from anthrax toxin as well as studies that demonstrate that inhibition of furin using hexa-D-arginine (D6R) protects mice from the toxicity associated with Pseudomonas exotoxin A as well as anthrax toxin. The ability of D6R to protect from the lethal effects of toxin was greatest when mice were pretreated with the drug. Since it is impossible to predict when an individual is going to be exposed to toxin, successful use of D6R in the field will require the development of a slow release formulation such that an individual is protected for 1-4 weeks. In specific aim 1, we will determine the pharmacokinetics of D6R in a rat model to determine the optimal dose of the peptide by subcutaneous (sc) route of administration. Experiments in specific aim 2 will focus on the determination of the maximum tolerated dose (MTD) as a single injection as well as toxicological assessment of therapeutic dose for a period of 4 weeks. Results of studies in aims 1 and 2 will be used in specific aims 3 and 4, to develop a slow release subcutaneous formulation such that protective concentrations of the drug within the plasma are maintained for a prolonged period (4 weeks). These studies will result in a formulation of D6R that will provide prolonged protection of an individual (e.g. a soldier in the field) from the lethal effects of anthrax toxin.